1. Field of the Invention
The present invention relates to a duplexer having a WLP (Wafer Level Package) structure.
2. Description of the Related Art
Conventionally, an elastic wave filter device having a WLP structure is widely used as a bandpass filter or other suitable components for a cellular phone device.
Japanese Unexamined Patent Application Publication No. 2011-172190 discloses an example of an elastic wave filter device having a WLP structure. In the elastic wave filter device of Japanese Unexamined Patent Application Publication No. 2011-172190, a support layer is formed on a piezoelectric substrate so as to enclose an elastic wave filter electrode portion formed on the piezoelectric substrate. A cover is provided on the support layer so as to seal a cavity in the support layer. A plurality of via hole electrodes are provided so as to penetrate the support layer and the cover. The elastic wave filter electrode portion includes a plurality of ground terminals to be connected to a ground potential, and the plurality of ground terminals are electrically connected to the plurality of via hole electrodes.
However, in the elastic wave filter device described in Japanese Unexamined Patent Application Publication No. 2011-172190, a large inductance is present between the ground terminals and the ground potential due to the plurality of via hole electrodes. Therefore, the isolation characteristics cannot be increased to a sufficient level.
In addition, in a conventional elastic wave filter device having unbalanced input-unbalanced output, in particular, the isolation characteristics cannot be enhanced to a sufficient level.